Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 181206 AAA
AFDPQR

Area Forecast Discussion...update
National Weather Service Portland OR
406 AM PST Thu Jan 18 2018

Updated marine and aviation

.SYNOPSIS...Strong low pressure off the British Columbia coast is
pushing a cold front slowly eastward across the Pac NW this morning.
This system is sending large surf toward the coast, which will
combine with high tides for some coastal flooding in low lying areas.
Meanwhile showers and isolated thunderstorms will persist through
this evening as cool air moves in aloft. Snow levels will lower below
the Cascade passes today, then likely remain below pass level for the
next several days. Cool onshore flow will cause showers to continue
through Saturday, then the next front will bring steadier rain and
mountain snow Sunday. Active weather looks to persist well into next
week.

&&

.SHORT TERM...Today through Saturday...Infrared satellite shows an
impressive mature low pressure system off the British Columbia coast,
where nearby buoys indicate surface pressure briefly fell below 970
mb as the low passed overhead. This system pushed a cold front across
the forecast area Wednesday afternoon, which has since crossed the
Cascades into eastern WA/OR. Perhaps more relevant to the coast is
the very large and dangerous surf this system is producing. The high
surf and high tides are combining to increase the threat of coastal
flooding, and a Coastal Flood Warning remains in effect through
midnight tonight. More about this impressive high surf in the marine
section below.

Based on surface pressure, satellite and lightning trends, it appears
a weak shortwave riding along the front caused a weak surface low to
form off the northern Oregon coast late Wed evening. This system
produced an impressive amount of lightning along and off the coast
from roughly Tillamook northward. Buoy 46029 reported a questionable
62 knot (71 mph) gust right as one of these thunderstorms moved over
it at about 1250 AM this morning. While the report is a bit of an
outlier, KLGX radar did pick up winds on the order of 55-65 kt at
4000 feet. Whether or not these actually surfaced will likely be for
only the buoy and perhaps a sea lion or two to know. Nevertheless,
the batch of showers associated with the developing low appears to be
moving onshore and off to the northeast, and should be long gone by
sunrise.

The core of cold air aloft associated with this system will move
onshore this afternoon and evening. The 00z/06z GFS convective
indices and NAM Bufr soundings show marginal instability moving
inland as 500 mb temps fall to -32 to -34 deg C. Past experience
suggests this may be a bit overdone, but even -30 to -32 deg C would
be enough to produce some weak instability with surface temperatures
in the upper 40s. Decided to add the mention of thunder inland this
afternoon and evening as a result. With freezing levels lowering, any
heavy showers or thunderstorms have the potential to produce small
hail; we have already had spotter reports of BB to pea sized hail
from the thunderstorms along the coast early this morning. If enough
sunbreaks occur to trigger strong updrafts at the surface, the
low-level shear profile appears favorable for some longevity to any
cells that develop, and a couple stronger cells with slightly larger
hail, gusty winds, and perhaps even a funnel cloud cannot be ruled
out today. Convective allowing guidance seems to suggest at least
isolated convection is likely this afternoon in the Willamette
Valley.

Cool onshore flow will cause showers to persist through Saturday,
with a shortwave providing some enhancement Fri night/Sat morning.
The risk of thunder looks less after tonight as temps warm slightly
aloft. Onshore flow will eventually weaken Saturday as our next
system approaches from the Pacific. This should result in a break in
the showers sometime late Saturday, but any break will probably be
brief with the next round of steady rain and Cascade snow arriving by
Sunday morning.  Weagle

.LONG TERM...Saturday night through Wednesday...Fairly zonal flow
will be in place over the central and eastern North Pacific, likely
keeping our weather active well into next week. Models show decent
agreement that a frontal system will move through the forecast area
Sunday, though there are some disagreements regarding the strength of
the front and whether or not split flow tries to develop off the West
Coast. The 00z ECMWF took an interesting turn as it develops a 993 mb
low off the Oregon Coast Tuesday, eventually curling it north and
moving it onshore near the Olympic Peninsula. The 00z/06z GFS were
more splitty with the Pacific jet energy, not showing as much phasing
between the northern and subtropical jets. If the 00z ECMWF pans out,
we will probably need to watch the Gorge for some sort of
frozen/freezing precip. Both the 00z ECMWF and the 06z GFS then come
back into agreement that much colder air comes in behind this system
Wednesday and Thursday. For now, went with fairly high PoPs in
general for the extended forecast period, with temps near to perhaps
a couple degrees below normal.Weagle

&&

.AVIATION...Mostly VFR today and tonight this evening. Isolated
thunderstorms may produce lightning, thunder, gusty winds, and small
hail through this evening. Flight conditions may temporarily reduce
to MVFR with any heavier showers.

PDX AND APPROACHES...VFR conditions with showers today and tonight.
Isolated thunderstorms are possible through this evening and may
result in brief and periodic MVFR conditions. ~TJ

&&

.MARINE...There are multiple marine hazards today as a low
pressure remains just offshore of the British Columbia coast and a
series of upper level shortwaves move over the region. Gales,
extremely large seas, and thunderstorms. Buoy 89, which is around 90
miles offshore peaked at 37 feet at 17 seconds around midnight last
night. The west swell should be reaching its peak heights for the
waters right now at 32-35 feet. The seas will subside to 20-25 feet
tonight, then below 20 feet by Friday afternoon. Gales should end to
the south this afternoon and to the north in the evening. Small
Craft Advisory winds will follow the Gales and persist through late
Friday night.

Another strong front is expected late in the weekend. Gales are
expected Saturday night. Models hint of possible storm force gusts
with the frontal passage Sunday morning. Seas will likely exceed 20
feet for a brief period on Sunday. Another front is likely on
Tuesday for another round of Gales and 20 ft+ seas. ~TJ

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Coastal Flood Warning until midnight PST tonight for Central
     Oregon Coast-North Oregon Coast.

WA...Coastal Flood Warning until midnight PST tonight for South
     Washington Coast.

PZ...Gale Warning until 7 PM PST this evening for Coastal Waters
     from Cape Shoalwater WA to Cascade Head OR out 60 NM.

     Gale Warning until 4 PM PST this afternoon for Coastal Waters
     from Cascade Head OR to Florence OR out 60 NM.

     Small Craft Advisory for Rough Columbia River Bar until 4 AM
     PST Friday.

&&
$$

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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. This area is
commonly referred to as the CWA or forecast area.


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